In aqueous microfiltration, such as the production of particle free water or solutions in the electronics and pharmaceutical industries, it is highly desirable to have membranes with as little leachable material as possible. Also, it is usually desired that the membranes be easily wettable by water or aqueous solutions. Therefore, membranes that are inherently water wettable (i.e., inherently water wettable as preformed membranes without having to be post-treated with a wetting agent). are often preferred over hydrophobic membranes post treated by a wetting agent. That is, it is common to manufacture hydrophilic membranes by adding a wetting agent, such as a surfactant, to preformed hydrophobic membranes. Upon use of the treated hydrophobic membranes, however, it is always possible that the wetting agent may be leached by the solution being filtered. Such leaching, in turn, may result in contamination of the filtrate.
At the present time, there are very few inherently wettable membranes that have been identified. Generally, these membranes are made of nylon. Examples of such membranes are disclosed in the U.S. Pat. Nos. 3,876,738 to Marinoccio et al, issued Apr. 8, 1975, and 4,340,479 to Pall, issued July 20, 1982.
Polymers which are inherently very hydrophilic are difficult to manufacture or have other drawbacks. Thus cellulose acetate membranes are limited in their heat and hydrolytic stability. Nylon membranes have to be made with solvent systems that are hazardous and hard to handle. Polymers which are even more hydrophilic such as polyvinyl alcohol, polyvinylpyrrolidone and the like are difficult or impossible to coagulate under practically feasible conditions because of their high affinity to water. Therefore, it is not generally convenient to make hydrophilic membranes out of highly hydrophilic polymers.
An ideal bulk polymer for a microfiltration membrane formation would be one which is inherently hydrophobic, to facilitate membrane formation, and would also impart desirable mechanical and thermal properties. However, the membrane made from such a polymer would also need to be inherently or easily water wettable by suitable means, e.g., by adding a wetting agent as noted above, such as a surfactant to facilitate aqueous filtration, but doing so in such a way as not to involve later aqueous leaching of the wetting agent and thus causing contamination of the filtrate.
This invention relates to a class of polymers surprisingly fulfilling these, apparently conflicting, requirements.
Polyamide, polyimide, polysulfone and polyether-sulfone polymers have been used in the preparation of membranes. For example, U.S. Pat. No. 3,816,303 to Wrasidlo, issued Jun. 11, 1974, discloses a process for desalination of saline water by reverse osmosis using a membrane comprising a film of poly(N-amido)imides having a specific formulation. The Wrasidlo patent describes a reverse osmosis membrane made from specific polyimide structures. The reverse osmosis membranes have a porosity significantly different than those found in microporous membranes. Fully aromatic polyamides have been used to make reverse osmosis membranes as exemplified by U.S. Pat. No. 3,172,741 to Jolley. Polyimides have been developed for gas separating membranes, as disclosed in Japanese document JP 58 08,514.
Polyimide polymers are usually insoluble and need to be formed in situ (that is, the membranes need to be synthesized) in the polyamic acid form and then heat treated to form the final polyimide membrane. Aromatic polyamides are not easily available as a pure bulk polymer and therefore usually need to be synthesized by a membrane producer. Unlike the aforementioned polymers, polyamide-imide is commercially available at reasonable cost and is soluble in commonly used solvents.
The U.S. Pat. No. 3,719,640 to Lee et al, issued Mar. 6, 1973, discloses linear polymers of polyamide-imides having a specific formulation containing a quaternizable nitrogen atom. The quaternizing of the nitrogen is pH dependent. When the nitrogen is quaternized, the polymer is hygroscopic and may be used as separatory membranes in such processes as desalination.
The U.S. Pat. Nos. 3,855,122 to Bourganel, issued Dec. 17, 1974, and 4,286,015 to Yoshida et al, issued Sep. 28, 1982 disclose membranes made from polyaryl ethersulphones having specific structural formulations.
The Bourganel patent discloses an asymmetric membrane having a porosity in the reverse osmosis/ultrafiltration range. The Bourganel patent only discloses membranes made from sulfonated polysulfone. The Yoshida et al patent also discloses asymmetric ultrafiltration membranes made from polysulfone.
The U.S. Pat. No. 4,240,914 to Iwama et al, issued Dec. 23, 1980, discloses a membrane and process for making the same made from an aliphatic polyimide polymer.
None of the aforementioned patents disclose an intrinsically hydrophilic membrane having a porosity in the microporous range.
The Japanese patent document to Shou 54-26283 describes a method of making apparently microporous membranes from polysulfone polymers with high molecular weight polyethyleneglycol as an additive. Common micro-filtration membrane characterization tests (such as bubble points, bacteria retention) are not reported and no distinction is made between the various possible polysulfones.
The present invention provides a membrane made of a hydrophobic polymer which, independent of pH, is surprisingly water wettable in a microporous membrane structure.